This disclosure relates generally to the field of measurements while wellbore drilling using a drill bit as an elastic wave energy source. More specifically, the disclosure relates to apparatus and methods for acquiring drilling vibration data created by drill bit interactions with the formation being drilled using sensors attached to the drill string, and in some cases the ground, and processing the acquired data to obtain properties, relative properties, or property indications of the rock formations being drilled using vibrations generated by interaction of a drill bit with rock formations.
Obtaining drilling vibration signals generated by interaction of a drill bit with rock formations during drilling of such formations is known in the art for the purpose of obtaining certain seismic properties of rock formations in the vicinity of the borehole. A method and apparatus for obtaining such signals and processing the signals to obtain seismic properties are described. In U.S. Pat. No. 4,926,391 issued to Rector et al. A generalized description of an apparatus disclosed in such patent is as follows. The apparatus includes a drilling rig and a rotary drill bit attached to the drilling rig for providing seismic waves traveling through the earth as the bit drills a formation. There is at least one seismic wave sensor spaced from the rotary drill bit in the earth for receiving signals traveling via direct seismic wave paths and signals traveling via seismic wave paths reflected by the subterranean geologic formation from the seismic waves provided by the drill bit. At least one reference sensor is located on or proximate to the drilling rig. A means is connected to receive the reference signal from the reference sensor and the drill bit generated signals from the at least one seismic wave sensor to distinguish the drill bit generated signals from interference signals by cross-correlating the reference and seismic wave sensor signals. The apparatus has a means connected to receive the reference signals either prior to or subsequent to their cross correlation for reference deconvolution or whitening. A means is connected to receive the cross-correlated reference and seismic wave sensor signals for eliminating rig generated energy from the reference signals. A means is connected to receive the cross correlated reference and seismic wave sensor signals from the rig generated energy eliminating means for separating the seismic wave sensor signals into a first group of the seismic wave sensor signals representing the drill bit generated seismic waves received by the at least one seismic wave sensor in the direct seismic wave paths, and a second group of the seismic wave sensor signals representing the drill bit generated seismic waves received by the at least one seismic wave sensor in the seismic wave paths reflected by the subterranean geologic formation. This method makes measurements corresponding to rock properties of formations located between the drill bit and the seismic sensors according to the ray paths of seismic signals in the earth.
Some properties of rock formations are not provided by seismic while drilling apparatus methods and apparatus known in the art. For example, some local mechanical properties of the of the rock formations at the interface between the drill bit and the rock formations, such as uniaxial compressive strength (UCS) density, Elastic Modulus are not provided. It is desirable to obtain such properties during drilling for wells used, as a non-limiting example, for blast holes drilled as part of construction of mining procedures. Having information about rock formation properties such as Elastic Modulus may assist in choosing appropriate blasting parameter (e.g., weight of, placement of and type of explosive) and in understanding the local geology of the formation being drilled for mine process optimization.